A description of a device for measuring the velocity of a fluid is available in document EP 0 347 096. In this known device, a measurement is taken of the time required for an acoustical signal to travel between two input-output transducers located at the ends of a cylinder through which the fluid is flowing.
If the distance between the two transducers is known, then the time taken for an acoustical signal to travel between the first transducer and the second, and between the second transducer back to the first, allows determining the velocity of the gas in the pipe.
However, this device has several disadvantages. The acoustical waves are emitted by the transducers with a conical radiation pattern. Apart from those waves actually on or near the centerline of the pipe, the waves are reflected off the pipe walls before reaching a receiver. These reflections lead to differences in the signal paths, hence to dephasing, depending on the transmission angle. Dephasing can lead to destructive interference at the receiver, which markedly decreases the intensity of the detected signal.
In addition, the radiation pattern implies increasing the frontal surface of the transmitter-receivers in order to increase the volume of signal received.
Finally, it follows that the electrical source required for supplying the transmitter-receivers must be powerful, which precludes the standard solution of an electric cell suitable for long periods of operation.
Notwithstanding, for use in a domestic gas meter, the device must be able to operate with very low electrical consumption, hence be compatible with such standard battery supplies, guaranteeing a service life of several years.